linux-kernel - Re: [PATCH] misc: Replace zero-length array with flexible-array member

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Date:   Thu, 27 Feb 2020 09:09:30 -0600
From:   Dimitri Sivanich <sivanich@....com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Tomas Winkler <tomas.winkler@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dimitri Sivanich <sivanich@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] misc: Replace zero-length array with flexible-array
 member

On Wed, Feb 26, 2020 at 04:22:40PM -0600, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>

Acked-by: Dimitri Sivanich <sivanich@....com>

> ---
>  drivers/misc/mei/hw.h            | 2 +-
>  drivers/misc/mei/mei_dev.h       | 2 +-
>  drivers/misc/sgi-gru/grulib.h    | 2 +-
>  drivers/misc/sgi-gru/grutables.h | 2 +-
>  4 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/misc/mei/hw.h b/drivers/misc/mei/hw.h
> index 8231b6941adf..b1a8d5ec88b3 100644
> --- a/drivers/misc/mei/hw.h
> +++ b/drivers/misc/mei/hw.h
> @@ -216,7 +216,7 @@ struct mei_msg_hdr {
>  
>  struct mei_bus_message {
>  	u8 hbm_cmd;
> -	u8 data[0];
> +	u8 data[];
>  } __packed;
>  
>  /**
> diff --git a/drivers/misc/mei/mei_dev.h b/drivers/misc/mei/mei_dev.h
> index 76f8ff5ff974..3a29db07211d 100644
> --- a/drivers/misc/mei/mei_dev.h
> +++ b/drivers/misc/mei/mei_dev.h
> @@ -533,7 +533,7 @@ struct mei_device {
>  #endif /* CONFIG_DEBUG_FS */
>  
>  	const struct mei_hw_ops *ops;
> -	char hw[0] __aligned(sizeof(void *));
> +	char hw[] __aligned(sizeof(void *));
>  };
>  
>  static inline unsigned long mei_secs_to_jiffies(unsigned long sec)
> diff --git a/drivers/misc/sgi-gru/grulib.h b/drivers/misc/sgi-gru/grulib.h
> index e77d1b1f9d05..85c103923632 100644
> --- a/drivers/misc/sgi-gru/grulib.h
> +++ b/drivers/misc/sgi-gru/grulib.h
> @@ -136,7 +136,7 @@ struct gru_dump_context_header {
>  	pid_t		pid;
>  	unsigned long	vaddr;
>  	int		cch_locked;
> -	unsigned long	data[0];
> +	unsigned long	data[];
>  };
>  
>  /*
> diff --git a/drivers/misc/sgi-gru/grutables.h b/drivers/misc/sgi-gru/grutables.h
> index a7e44b2eb413..5ce8f3081e96 100644
> --- a/drivers/misc/sgi-gru/grutables.h
> +++ b/drivers/misc/sgi-gru/grutables.h
> @@ -372,7 +372,7 @@ struct gru_thread_state {
>  	int			ts_data_valid;	/* Indicates if ts_gdata has
>  						   valid data */
>  	struct gru_gseg_statistics ustats;	/* User statistics */
> -	unsigned long		ts_gdata[0];	/* save area for GRU data (CB,
> +	unsigned long		ts_gdata[];	/* save area for GRU data (CB,
>  						   DS, CBE) */
>  };
>  
> -- 
> 2.25.0
>